1. Field of the Invention
This invention relates to an internal voltage generating circuit that is provided inside a semiconductor device and generates an internal voltage to be supplied to an internal circuit of the semiconductor device from an external voltage inputted from the outside.
2. Description of the Related Art
As a technique related to this type of internal voltage generating circuit, one is known which has been disclosed in, for example, Japanese Patent Application Laid-Open No. 6-96596 (Laid-Open Date: Apr. 8, 1994). FIG. 7 shows one example of an internal voltage vs. external voltage characteristic of a conventional internal voltage generating circuit. In FIG. 7, the internal voltage indicates such a constant voltage characteristic that when the external voltage ranges from 0 to a voltage VN (first voltage section or range), the external voltage is outputted as the internal voltage and when the external voltage ranges from the voltage VN to a boundary voltage VT (second voltage section or range), a constant voltage is outputted regardless of the external voltage. Further, the internal voltage indicates such a variable voltage characteristic that a voltage is outputted which vertically rises at the final stage of the second voltage range and linearly rises from the voltage that has risen at the final stage of the second voltage range in a section or range (third voltage range) in which the external voltage becomes greater than or equal to the boundary voltage VT.
With the objective of performing a screening test for an initial failure and a reliability test on newly-developed semiconductor devices, a burn-in test for applying a source voltage higher than normal specifications to manufactured semiconductor devices so as to activate them under high temperatures is applied to each manufactured semiconductor device. During the bum-in test, the semiconductor device is activated in the third voltage range. During the normal operation on the other hand, the semiconductor device is activated in the second voltage range. Whether the semiconductor device should be activated in the second voltage range or the third voltage range, is controlled according to the level of an applied external voltage. Further, the switching between the voltage ranges is carried out by changing the level of the external voltage.
However, in the conventional internal voltage generating circuit, when fluctuations occur in the external voltage due to the production of noise or the like in the vicinity of the boundary voltage VT corresponding to a point for switching from the second voltage range to the third voltage range or from the third voltage range to the second voltage range, the section or range of the internal voltage is not suitably set to either the second voltage range or the third voltage range and hence becomes unstable, thus resulting in the output of an unstable internal voltage from the internal voltage generating circuit